In large electric generators, a stator assembly includes stator coils which are retained within an armature slot. The stator coils may be held in place by stator wedges, which are typically inserted within a pair of opposed parallel grooves on either side of the armature slot. Top ripple springs can be inserted between the stator wedges and the stator coils to maintain a positive load on the stator coils and to thereby maintain a consistent pressure or preload on the stator coil. In addition, side ripple springs may be inserted along the side of the stator coils in engagement with the armature slot walls to reduce vibration of the stator coils in the tangential direction.
A common problem associated with stator coils is that they can become loose within the armature slots. It has been found that loosening of the stator coils within the armature slots may lead to relative movement between the stator coils and the armature slot and an increase of levels of vibration of the stator coils. These conditions may cause deterioration and failure of stator coil insulation and stator core laminations. Further, it is possible that tangential vibration of the stator coils within the armature slot may cause electrical discharges between the stator coils and the stator core, which may result in spark erosion of those components. These undesirable results may necessitate replacement or time consuming and difficult repair procedures, such as a stator coil rewinding process.
Tests have been developed to estimate the tightness of stator wedges and top ripple springs within the stator assemblies. One such test, disclosed in U.S. Pat. No. 6,631,335, involves exciting a vibration in a stator wedge of a stator assembly, which includes the stator wedge, one or more stator coils, and optionally ripple springs, which is disposed in an armature slot. The vibratory response of the stator assembly is measured and compared to previously saved vibratory responses of stator assemblies to estimate the tightness of the stator assembly within the armature slot.
Another such test, disclosed in U.S. Pat. No. 5,012,684, discloses a wedge block having a hydraulic cylinder, wherein the wedge block is installed within an upper groove of a pair of parallel grooves in an armature slot or a stator slot, The hydraulic cylinder is activated to press down on a load plate, which in turn contacts a slot wedge within the other of the parallel grooves. This in turn compresses a top ripple spring between the slot wedge and the stator windings. A load cell measures the force required to compress the top ripple spring, while four linear variable differential transformers measure the amount of displacement of the slot wedge. These two measurements are then used to derive a measurement for the tightness for the combined structure of the slot wedge and the top ripple spring.
Tests such as these are performed with the stator wedges in place, and may be effective at determining the tightness of stator wedges and top ripple springs within a slot. However these types of tests can be inconclusive with respect to side-to-side or tangential stator coil tightness, and do not provide a measurement specific to vibration of the stator coils themselves within the armature slots. Moreover, as stator coils implemented in current generator designs become increasingly thinner with an associated increase in flexibility in the tangential direction, testing based on a vibratory response of the stator wedge and ripple spring structure installed in the slot may become less precise when estimating component tightness within the slots.
Current methods of determining a relative tangential tightness of stator coils within stator slots include the use of “feeler gauges” that are probed into the space between the stator coil and a wall that defines the armature slot. The feeler gauge provides the tester with a feel for the approximate width of the space. Since no direct reading of the tangential tightness of the stator coil within the armature slot is measured, the precision of the readings supplied by the feeler gauges is low.